Rate-responsive or demand-type cardiac pacemakers are widely available in the industry. In contrast to programmable fixed rate pacemakers, rate responsive pacemakers implement one or more sensors for monitoring and indicating various attributes of a patient, including both a patient's physiological needs, as well as detected physical activity levels. Various devices and algorithms are known in the art for determining the outputs from sensors and controlling the pacing rate as a function of the sensor outputs. These various prior art devices attempt to account for time delays when sensing attributes of a patient's need, and for false or inaccurate sensor outputs, such as the output provided by an accelerometer when the patient travels over a rough road in an automobile.
Many parameters can be continuously sensed and responded to by the pacemaker via the sensors when determining the optimal pacing rate, given the conditions of the patient as sensed by the sensors. Such parameters include sensing the QT interval, which is the time interval between a delivered pacing stimulus and the subsequent evoked T-wave. Activity sensors such as accelerometers, as well as sensors for obtaining a patient's respiration rate, thoracic impedance changes, venous blood temperature, PH levels, oxygen saturation, and heart stroke volume are all known in the prior art for use with pacing circuitry.
Generally, no one parameter is adequate standing alone as a basis for determining a delta pacing rate of a rate responsive pacemaker. Delta pacing rate is defined as the increased pacing rate above a predetermined minimum pacing rate. While each sensed parameter is helpful for ascertaining a patient's present condition when determining the optimal pacing rate, prior art devices have recognized the advantage of sensing more than one parameter when determining the optimal pacing rate. One approach is to use a first sensor to qualify a second sensor. The pacing rate is thus based on one sensor as long as the second sensor "qualifies" the rate increase as legitimate. This greatly reduces the potential benefits of a two sensor pacemaker since for the majority of the time the pacing rate is based only on one sensor. A second approach blends the two sensors at a constant percentage, such as 50%. This method simply dilutes the input of one sensor with the other and thereby reduces the worst and the best characteristics of each sensor.
Other methods of combining sensor inputs within a pacemaker are non-programmable, or at the very best, rigid and allow no fundamental changes in the method by which the sensors are combined. This makes adaption of clinical investigations leading to improved blending algorithms much more difficult. In addition, patients having pacing devices using old blending algorithms cannot take advantage of improved blending algorithms without replacement of their present pacemaker.
U.S. Pat. Nos. 4,688,573 and 4,782,836 to Alt teach a rate adaptive cardiac pacemaker responsive to patient activity and temperature. This invention teaches using two different algorithms, one exclusive of the other, depending on whether the output of a temperature sensor has exceeded a predetermined temperature threshold. The two algorithms are characterized as an exercise algorithm, and an algorithm for patient inactivity. The primary sensor is an activity sensor such as a piezoelectric crystal for detecting movement of the patient.
U.S. Pat. No. 5,065,759 to Begemann et al. teaches a pacemaker with optimized rate responsiveness and method of rate control. The algorithm taught is based upon implementing two sensors, one designated as having a fast sensor rate, and the other designated as having a slow sensing rate. One sensor provides a parameter taken as the primary control parameter, and the other provides a parameter which is converted into corresponding units so as to be comparable for control purposes. An algorithm compares the difference between the detected sensing rates, and determines an adjust rate difference (drift). The pacing rate is established by incrementing the pacing rate at a rate depending upon this drift rate and a predetermined factor "C". Thus, gradual rate changes are achieved through incremental adjustments, and are based on inputs alone and without using feedback. This device implements two sensors, wherein one may remain deactivated until a sufficient magnitude indicating the undertaking of physical exertion. Upon activation of the first sensor, a linear increase in the pacing rate is performed at a predetermined rate but which is not to exceed a predetermined upper threshold. Similarly, when the activity sensor senses a reduction of physical activity, the pacing rate will fall progressively but not below a minimum rate. Thus, one sensor serves to determine whether a pacing rate should be increased or decreased, and the other sensor serves to modify the upper and lower pacing limits. In effect, the second sensor serves to modulate the upper limits.
U.S. Pat. No. 5,063,927 to Webb teaches a rate responsive pacemaker which generates a pacing signal as a function of two separate sensors. If a change of a pacing is to be performed, the delta pacing rate is a predetermined and fixed increase or decrease, such as one beat/minute/second, and the delta pacing rate is not variable or dependent on variables.
U.S. Pat. No. 4,867,161 to Schaldach teaches a cardiac pacemaker implementing matrix logic for determining a pacing rate. The matrix provides a look-up table based strictly on inputs from sensors when determining a delta pacing rate, and doesn't implement feedback such as using a current pacing rate as an input variable to the matrix. Thus, transitions between a fast pacing rate and a slow pacing rate are not necessarily smooth, and may be noticed by the patient.
Accordingly, an improved rate responsive pacemaker having two or more sensors for sensing parameters of a patient is desired which simultaneously uses and blends the sensed parameters when establishing the optimum pacing rate. An algorithm which provides for a smooth transition when altering a pacing rate by utilizing the pacing rate established by previous pacing pulses is desired. To utilize the benefits of each sensor, one should not be excluded from the other except when a sensor output is deemed invalid.